There are some weird dynamics surrounding Obama's speech tonight. All of them, I think, point to a more subdued and workmanlike performance than many Democrats are hoping for.
(1) Obama delivered the speech of his career in 2004. Is it possible to give a better performance? Probably not, especially considering how the myth of that speech has grown. Obama may decide to give a different kind of speech tonight — not a barnburner like in 2004, but instead a deeply serious and policy-heavy speech that suggests Obama is ready to lead in difficult times. It wouldn't serve him well to try and fail to recreate 2004's magic.
(2) Today is the 45th anniversary of Martin Luther King Jr.'s "I Have a Dream" speech. Does Obama want to show that up? Does he want to risk the appearance of showing that up?
(3) The McCain campaign is slamming Obama daily for his "celebrity" status. Anything that works 75,000 adoring fans into a tizzy is just fodder for opposition ads. If Obama can do avuncular, this might be the night to do it. (Admittedly, there may be little Obama can do here. There will be 75,000 people there. They will go nuts.) The challenge is to do avuncular without failing the media's (and his supporters') massive expectations.
I think the campaign originally saw this as a culminating moment for the Obama movement. He would speak on the anniversary of King's speech in front of 75,000 people. King was a vanguard for change, creating a mass movement that followed behind him and revolutionized the country. If you remember some of Obama's early rhetoric, there was a strong strain in it that suggested Obama saw himself as the same sort of figure. But the idea of a "movement" has been turned from a positive into a negative. What was once universally acknowledged as a wonderful development in American politics — young people, minorities, and others traditionally uninterested in public affairs finally finding inspiration — has been turned into a liability.
Once upon a time, Obama wanted to be carried into office on the back of a movement; today, he may do what he can to pretend one doesn't exist.